Make the session cookie name configurable for a staging deployment

Groundwork for testmodul.qrmaster.net, a second stack running the `test` branch on a real
qrmaster.net subdomain.

Production scopes its session cookie to .qrmaster.net, so the browser sends it to every
subdomain including staging. With both environments naming the cookie `userId`, the
browser holds two cookies of the same name and cookies.get() picks one arbitrarily -
staging logins would look randomly signed-out. AUTH_COOKIE_NAME lets staging pick
`userId_test` instead. Production keeps the `userId` default; changing it there would
invalidate every existing session.

Wired getAuthCookieName() into the six places that named the cookie literally. The account
deletion route now expires both the host-only and the domain-scoped variant like the logout
route already does, instead of a single cookies().delete() that would leave the other one
behind.

NEXT_PUBLIC_WWW_URL and NEXT_PUBLIC_APP_URL become build ARGs so the same image can be
built pointing at the staging host - the defaults keep a plain production build byte
identical to before. Like COOKIE_DOMAIN these must exist at build time, because process.env
is inlined into the Edge middleware bundle.

robots.ts now serves Disallow-all unless NEXT_PUBLIC_INDEXABLE is true. Staging otherwise
returns the production robots.txt and invites crawlers to index a duplicate of www.

docker-compose.test.yml is the staging overlay. Two things it must get right, both verified
against `docker compose config`:

- db and redis need `networks: !override`. Compose MERGES the networks mapping from the base
  file, and since qrmaster-network is external and shared, a plain list left them attached
  to it - `db` would then resolve to two containers and staging could read and write the
  production database.
- The web entrypoint is replaced so `prisma migrate deploy` never runs. prisma/migrations
  stopped in April 2026 and the schema has moved on through manual SQL since, so applying
  them to a fresh database would build a stale schema. Staging gets its schema from
  `pg_dump --schema-only` against production instead.

Verified: tsc clean, production build succeeds, and the merged compose config confirms
staging keeps db/redis off the shared network while production resolves unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

env.example

@@ -21,6 +21,13 @@ NEXTAUTH_SECRET=your-secret-key-here-change-in-production
 # between www.qrmaster.net and app.qrmaster.net. Only honoured when NODE_ENV=production.
 COOKIE_DOMAIN=
 
+# Name of the session cookie. Leave empty in production and development (defaults to
+# `userId`). The staging deployment on testmodul.qrmaster.net must set its own name, e.g.
+# `userId_test`: production scopes its cookie to .qrmaster.net, so the browser sends it to
+# every subdomain, and two cookies with the same name make the lookup ambiguous.
+# Changing this in production logs out every user.
+AUTH_COOKIE_NAME=
+
 # Host split: marketing/SEO on WWW, the logged-in app on APP. Keep both pointing at the
 # same origin locally so nothing redirects across hosts in development.
 # In production: NEXT_PUBLIC_WWW_URL=https://www.qrmaster.net